Chromate-fluoride type coating solutions and method of treating metal surfaces therewith



Gaillard W. Dell, Detroit, Mich.,. assignor to Parkcif Rust Proof Company, Detroit, Mich, a corporation of Michigan No Drawing. Application January 31, 1957 Serial No. 637,350

14 Claims. (Cl. 148-616) This invention relates to the art of producing chemically formed corrosion resistant and paint-base coatings on metallic surfaces from chromate-fiuoride type coating solutions.

It is known that chromate-fiuoride coating solutions of the type which will be more particularly described hereinafter and to which this invention generally relates are effective for coating surfaces of aluminum and alloys thereof but do not produce commercially satisfactory coatings on iron and ferrous alloy surfaces.

Accordingly, it is an important object of the present invention to provide improved chromate-fluoride type coating solutions which are multi-purpose coating solutions effective in forming entirely satisfactory corrosion resistant and paint-base coatings on widely different metallic surfaces, particularly on surfaces of either aluminum or iron and alloys of each.

Another important object of the invention is to provide an improved method of treating widely different metallic surfaces to form thereon corrosion resistant and paintbase coatings of increased coating weight.

The above and related objects will become apparent during the course of the following description considered in its entirety.

In accordance with the present invention it has now been found that certain condensed phosphate compounds, when incorporated in well defined proportions in chromate-fiuoride coating solutions of the type to be described, unexpectedly broaden the range as to the metallic surfaces on which satisfactory corrosion resistant and paintbase coatings can be formed. The improved chromatefluoride coating solutions which constitute the subject matter of the present invention contain one or more condensed phosphate compounds and have been found to produce coatings on surfaces of iron and iron alloys in addition to those of aluminum, and even on surfaces of magnesium and magnesium alloys which are commercially acceptable from the standpoint of bare corrosion resistance and receptivity for paint, lacquer, enamel, varnishes and other siccative coatings. It has also been found that the coatings produced from the condensed phosphate compound-containing chromate-fiuoride solutions form particularly good base coatings for various adhesives commercially used for applying plastic sheet film to metal-surfaces.

The condensed phosphate compounds which are additives for the chromate-fiuoride type coating solutions in accordance with the present invention are sometimes referred to as complex or sequestering phosphate compounds but the term condensed phosphate compounds is the one used herein. Briefly, the condensed phosphate compounds that have been found to be useful are those which yield the condensed phosphate radical in solution and which are the alkali metal condensed phosphate salts which term includes the ammonium salts and the condensed phosphoric acids from which such salts are derived. From a standpoint of availability and economy the alkali metal condensed phosphate salts are preferably used. The condensed phosphoric acids are formed 4 by condensation of phosphoric acid with elimination of the water and the term condensed phosphate compound includes only thosecondensed phosphates of the type just mentioned in which the mole ratio of cationic oxides to anionic oxides is greater than 0 and less than 3. All phosphate compounds can be represented stoichiometrically as combination of oxides. Thus two molecules of Na HPO can be written as 2Na O-H O-P O and similarly two molecules of Na P O can be written as 5Na O-3P O The mole ratio of cationic oxides (such as Na O and including H O of composition) to anionic oxides (P 0 defines any given phosphate compound, and as previously indicated, this ratio must be greater than 0 and less than 3 for the condensed phosphate compound usable for the purposes of the present invention. The condensed phosphate compounds of the type with which the present invention is concerned generally fall within certain fairly well recognized groups, examples of which in the order from the lower to the higher condensation product are: the ultraphosphates; metaphosphates; polyphosphatcs; and the pyrophosphates. Orthophosphate compounds are those in which the mole ratio of cationic oxides to the anionic oxides is 3. Condensed phosphate compounds derived from phosphoric acids which in turn are formed by the elimination of approximtely one molecule of water from one molecule of orthophosphoric acid are the metaphosphates and the mole ratio for the metaphosphates is l. Condensed phosphate compounds in which the mole ratio of cationic oxides to anionic oxides is less than 1 and greater than 0 are the ultraphosphates. Condensed phosphate compounds derived from phosphoric acids which in turn are formed by the elimination of approximately one molecule of water from two molecules of orthophosphoric acid are the pyrophosphates and the mole ratio of cationic to anionic oxides for the pyrophosphates is 2. Condensed phosphates in which the mole ratio of cationic oxides to anionic oxides is less than 2 and greater than 1 are the polyphosphates. Condensed phosphates in which the mole ratio is less than 3 and greater than 2 are obtainable but do not fall within any recognized group. Reference is made to the Encyclopedia of Chemical Technology, vol ume X, pages 404 to 510, published by Interscience Encyclopedia, Incorporated, 1953, for a more complete description of condensed phosphate compounds. As previously indicated, the condensed phosphate radicals are preferably introduced through the alkali metal condensed phosphate compounds. Specific examples of condensed phosphate compounds that may be employed in the practice of the invention are tetra-sodium pyrophosphate, disodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate, and the respective condensed phosphoric acids from which these salts are derived. Potassium and ammonium condensed phosphate salts cor responding to the sodium salts enumerated may likewise be used.

The amount of condensed phosphate compound as defined above that is added to the chromate-fiuoride coatmg solutions of the type to be described must be controlled within certain critical proportions to effectively coat both surfaces of iron and aluminum and alloys of each. The concentration of the different ingredients throughout the specification and in the appended claims is expressed in percent w./v., which refers to weight per unit volume, i. e., the specific gravity of the solutions is taken to be 1.0. In the case of the specified condensed phosphate compounds their concentration is conveniently expressed as percent by weight based on the condensed phosphate of sodium compound, The differ- 3 ent condensed-phosphateof-sodium compounds are substantially equally effective onastraight weightbasis and where condensed phosphate compounds other than the sodium salt are employed, the amount of these other compounds is adjusted to-produce'the same effect as the amount of condensed phosphate'of sodium compound within the critical ranges hereinafter-set forth. When the concentration ofthe'condensed phosphate compound exceeds about 0.5% by weight based on condensed phosphate of sodium compound, dusty coatings onsurfaces of iron, steel and other ferrous alloys are formed which are useless froma practical standpoint as a base for siccative coatings and for bare corrosion resistance. -On surfaces of aluminumand aluminum base alloys no benefits with respect to bar-e corrosion resistance and paintbase qualities -were obtained when the concentration exceeds themaximum-indicated. The concentration of the condensed"phosphate compound-maybe .as low as that which produces a-beneficial effect "on resistance to corrosion and paint-base qualities-butE-from a commercial standpoint it-has been =-found that very -little ;-benefit is produced when the concentration-ofthecondensed phosphate compound-is -below about 0.-l-% based on condensed phosphate of sodium-compound. Preferably, the con-densed phosphate-compound is added in amountsto produce the same effect as that observed-fromsolutions containing about 0.05% by weight-of:condensedphosphate ofsodium compound. Within the range of concentrations for condensed sodium phosphate compound setforth the coatings on iron and aluminum surfaces range from fair to excellent as faras bare'corrosion resistan'ce and receptivity to siccative coatings are-concerned. By bare corrosion resistance'is meant the .resistance of the coated metalvsurfacesafter treatment in the working baths and does not-refer to -bare, untreated metal surfaces. lnmany instances'metal surfaces are employed in the condition in whichthey areremoved from the working baths of the-present .inventionwithout-the sub sequent application of .lac quers paints, or other additional protective coatings. =For example,-many aluminum structural parts are employeduwithoutbeing painted and for this purpose- 'itis'desirable to haveonthe surfaces .of the aluminum-parts'acoating which protects it from corrosion inthe chemically coated but unpainted condition and bare corrosion resistance is a measure of this type of protection.

The 'chromate-fluoridetypesolutions that are improved by the addition in the-proporticns previouslyset forth of one'-or:more of' -the condensed'phosphate compounds are s aqueous .acidic solutions which contain -hexavalent chromium and the fluoride radical and preferably the fluoborate radical. T The concentration -'of "hexavalent chromium in 'the' coatingbaths constitutingthe subject matter of the present invention can-vary-from about 0.08% to about2;5% Ci-O "Preferably-the concentration of hexavalent chromiumis maintained between' 0.3% and 0.6%, .CrO .by weight. 'Chromic acid or alkali metal dichromateszincluding ammonium 'dichromate or dichromates. ofzzinc, calcium, cadmium or aluminum can be used to introduce hexavalentchromium. The concentration .of the fluoride ion can vary substantially andyet produce satisfactoryresults. The fluoride whichis-prescut in the solution may be .-presentin the form of 'free fluoride or .as. a complex fluoride of one or more-of the metallic ions which are also present in thesolution. For example,-the fluoride .may .be present as a complex of aluminum, titaniumafiuobnrate,:iron, etc. The quantity of fluoride, ion present Linithe, solution; necessarilywill tend to increase asv the-solution is, aged= ;and-wil1 ultimately, in any continuously operating-solution; reach azsubstantially constant value whenabalance betweenthe'quantity used in theformation of thecoating and thatc-arriedout on the work is reached. In;the operation-ofthese solutions itis customary.to .analyze the solution for total fluoride and for satisfactory operation, :the total flufiride content may vary between about 0.01% and about 2%,

expressed as weight percent ofsodiumbifluoride. It is preferable to maintain the concentration of the free fluoride ion between 0.01% and about 0.1% since these quantities of free or active fluoride are adequate to obtain good coating action. The fluoride ion can be introduced into the coating solution by the addition thereto of soluble, simple or complex fluorides such as hydrofluoric acid, hydrofluosilicic acid and the salts thereof, sodium bifiuoride, or the combination of sulfuric acid and sodium fluoride in proportions to produce the desired concentration of fluoride ion.

The presence of fluorborate ions in the solution has been observed to have the effect of broadeningthe range of concentration of the chromate and fluoride ions which may be present in order to obtain satisfactory coatings. For this reason the preferred solutions of this invention include the fluoborate ion. When fluoborate is employed,- the quantity -may satisfactorily extend as high as about 1.6% with a preferred range of fluoborate being 0.65%0.70%, all percentages of fluoborate ion being expressed as weight percent of sodium fiuoborate. The 'fluoborate ion is preferably introduced through the use of alkali metal fluoborates although fiuoboric acid or the combination of hydrofluoric acid and boric acid and their salts rnay be satisfactorily used. The increase in the range of the chromate and fluoride ion contents of working baths containing fluoborate .in comparison to thosecontaining the fluoride and chromate ions only is illustrated by the following. In a bathcontaining 1 gram per liter NaHF and chromic acid, good coatings were obtained at CrO concentrations between 8 and lOgrams/liter. In the presence of 12.5 grams/ liter NaBF and 1 gram/liter NaHF good coatings were obtained at CrO concentrations of 6-12 grams per liter. It was also found that good coatings were obtained in baths containing 8 grams/liter Natl-1P 12.5 grams/liter NaBF and 24 grams/liter CrO whereas in the absence of NaBF good coatings could not be obtained at concentrations of CrO higher than 20 grams per liter and NaHF concentrations higher than 5 grams/liter.

A feature of the invention is the provision of a coating solution which contains in addition to all of the other ingredients previously enumerated within their respective proportions, potassium titanium fluoride and/or potassium zirconium fluoride which solution has effect of eliminating dusty coatings particularly on ferrous metal surfaces at the higher concentrations of ingredients. Potassium titanium fluoride and/or potassium zirconium fluoride when incorporated in thensolutions which constitute'the present invention, .should be added in amounts up to about 0.1% by weight. Working baths were; prepared containing tetrasodiurn py-rophosphate in amounts from 0.05% to 0.5%, together with 0.3% CrO 0.67% sodium fluoborate and 0.075% sodium bifiuoride and were modified by the addition of 0.01%, 0.02%, 0.03%, and 0.04% by weight per volume potassium titanium fluoride. The baths in addition contained 0.75% weight per volume nickel sulphate and approximately 0.5% weight per volume ferrous sulphate. A substantial reduction'in the amount of dust produced on hot rolled steel panels processed in' the above modified baths was observed because of the presence of potassium titanium fluoride in the amounts mentioned.

The range of proportions-0f CrO fluoride and :the condensed phosphate compounds as above set forth operate satisfactorilyin anaqueous acid solution. The acidity of the solution is not critical but will necessarily vary with the proportion of chromic acid which is present. For the above given variations of ingredients a Working bath may have a free acid content varying between about 2 /2 points to about 35 points and preferably about 2 /2 toabout 8 points. The free acid may be adjusted during operation with nitric acid to obtain the desired level. The term free acid points refers to'the, number of; ml.

seesaw 5 of N/ 10 sodium hydroxide solution which is required to titrate a 10 ml. sample of the bath to a dark green color using bromcresol green as the indicator.

The coating solutions of the present invention are particularly valuable in that they produce satisfactory coatings on widely different metallic surfaces such as iron, steel, nodular iron and iron whichhas been galvanized, and articles of zinc or magnesium and of alloys of each. Briefly, the method of the present invention comprises treating metallic surfaces with chromate-fluoride coating solutions of the type previously indicated which contain one or more of the specified condensed phosphate compounds for a time sufficient to produce thereon corrosion resistant and paint-base coatings. The coating solutions of the invention may be applied to the metal surfaces to EXAMPLE I Gms./liter CrO 3.7 NaI-IF 0.75 Condensed phosphate of sodium compound 1.0

Three baths were prepared using concentrations of ingredients as above and in each the chromate radical was introduced through the use of chromic acid. The condensed phosphate of sodium compound employed for each different bath was sodium tripolyphosphate, sodium hexametaphosphate, and tetra-sodium pyrophosphate and the baths had free acid contents respectively of 4.8, 6.1 and 5.3. Panels of 3S and 2431 aluminum alloy were cleaned, warm-water rinsed and processed for five minutes in each of the above baths. The coated 3S panels were dried and subjected to a salt-spray for the time indicated in the table below which lists the average of the results of the series of panels treated in each bath and for bare metal. Potassium and ammonium salts corresponding to each of the enumerated sodium condensed phosphate salts produce comparable results.

In the salt-spray test, the lower the number the better the corrosion resistance. The number is excellent, while the number 5 represents rusting of the panel as far as commercially practical use of the coating is concerned.

Cold rolled steel panels and panels of 3S and 248T aluminum alloy were cleaned, rinsed and processed in the above baths for five minutes. Coatings which were sat- ,isfactory for bare corrosion resistance were produced on both the steel and aluminum surfaces with the coating on the aluminum the lighter in color. The bath of Example I-B above was modified in that the concentration of the tetrasodium pyrophosphate was increased in increments. Satisfactory coatings on both aluminum and steel surfaces continued to be produced upon the incremental increases in concentration up to about 0.5% of tetra-sodium pyrophosphate. When this concentration is exceeded, extremely dusty, non-adherent coatings are produced on steel surfaces and aluminum surfaces show no indication of any coating.

EXAMPLE II Sets of panels of 38 aluminum and aluminum-killed steel were cleaned and processed in the following bath for 5 minutes. Panels of M44H magnesium were similarly cleaned and processed in the same bath for 2 /2 minutes. All of the panels were rinsed in cold water and given a final hot water rinse. The composition of the bath is given below. The chromate radical was introduced by the use of chromic acid.

' Gms./liter CrO 3.7 NaHF 0.75 Tetra-sodium pyrophosphate 1.0

Uniform coatings were obtained on each of the panels. The coated panels were then laminated with 0.017 inch thick vinyl plastic sheets by covering the surfaces thereof with a 0.10 mil thick layer of conventional plastic adhesive and then laying over the vinyl plastic sheet and subjecting the laminated panels to a pressure of 100 lb./sq. in. at 350 F. for 10 minutes. Bare metal control panels were similarly laminated with vinyl plastic sheet. Pull strips A inch wide were then cut into the adhered plastic sheet and the adhesion of these strips to the coated metal surfaces and to the metal bare surfaces in the case of the control panels was measured in a con ventional Scott tester. On the bare metal control panels the plastic adhesive pulled loose from the surface of the panel with the plastic sheet, but on the treated panels the adhesive remained adhered to the coated metal surface and the plastic film pulled loose from the layer of adhesive.

' EXAMPLES III THROUGH V EXAMPLE III Gms./liter CrO 3.7 NaHF 0.75 NaBF 6.7 Tetra-sodium pyrophosphate The panels of the cold rolled steel and 24 ST aluminum were processed for five minutes and the panels of zinc grip and M44-H magnesium were processed for 2 /2 minutes and 1 minute respectively.

EXAMPLE IV Gms./liter' CrO 1.85 NaHF 0.38 NaBF 3.35 Tetra-sodium pyrophosphate 0.25

The panels of 24 ST aluminum and zinc grip were processed for five minutes and the panels of cold rolled steel and magnesium were processed for 2% minutes and 1 .minute respectively,

7 EXAMPLE V Gms/liter- CrO 0.93 NaHF 0.19 NaBF 1.68 Tetra-sodium pyrophosphate- 0.13

The panels of cold rolled steel, 24 ST aluminum and zinc grip were processed for 5 minutes and the panels of magnesium were processed for one minute.

Satisfactory coatings generally were produced on all of the panels of a set processed in each of the above baths. The baths in each of the abovethree examples were modified by omitting the tetra-sodium pyrophosphate and no coating at all was produced on the cold rolled steel panels while coatings were produced on the aluminum panels.

Some of the coated panels treate'din' each of the above baths were tested for their bare corrosion'resistance, i. e., their ability to withstand corrosion in their as-processed condition without additional protective coating. Others of the coated panels were painted and tested for their paint receptive properties bys'ubjecting't-he painted panels to salt-spray and humidity chambers. The painted panels were also tested for adhesion. The tests conformed to generally accepted standard procedures fordetermining comparative ratings for adhesion, paint-base properties, and corrosion resistance. The results showed that coatings on both the cold rolled steel and aluminum panels were excellent paint-base coatings. The bare corrosion resistance on the coated steel panels was satisfactory but not as good generally as on the aluminum panels. On the panels of zinc grip and magnesium uniform coatings were formed which tested excellent in adhesion. The paint-base characteristics of the coatings on the zinc and magnesium panels ranged from fair to good with some panels considerably poorer than others in this respect.

EXAMPLE VI 7 CrO gms./liter 3.7 NaHF gms./liter 0.75 NaBF gms./liter 6.7 Sodium tripolyphosphate gms./liter 1.0 Free acid points 4.9

EXAMPLE VI-A CrO gms./liter 3.7 NaHF gms./liter 0.75 NaBF gms /liter 6.7 Sodium tripolyphosphate gms./later 2.0 Free acid points 4.5

EXAMPLE VII CrO gms./'liter 3 .7 NaHF g'ms./liter 0.7-5 NaBF gms /lite1. 6.7 Tetrasodiurn pyrophosphate gms./liter 1.0 Free acid points 4.7

EXAMPLE VII-A CrO ms /liter 3.7 NaI-IF gms./liter .75 NaBF gms./liter 6.7 Tetrasodium pyrophosphate gms./liter 2.0 Free acid points 3.9

EXAMPLE VIII= CrO gms./liter 3.7 NaHF gms./liter .75 NaBR; gms./liter 6.7 Disodium' pyrophosph-ate gms./liter 110 Free acid points 5.7

EXAMPLE VIII.A

CrO gms./liter 3.7 NaHF gmsa/literu .75 NaB'F' gms /liter; l 6.7 Disodium pyrophosphate gm's/liter 2.0 Free acid points 6.0

&

Sets of 4" x 6" x 24- gauge cold rolled steel panels. and 4" x 6" x 0.032" 24 ST aluminum panelsv were precleaned by a butyl Cellosolve wash, a vapor degrease treatment and then wiped dry with a cloth. The panels were then subjected to conventional rinsing steps and then processed at temperatures from 75 to F. for 5 minutes in the working baths of the above examples. To simulate mixed production the aluminum and cold rolled steel panels were processed by following one load of aluminum panels with one load of steel panels in each of the baths until all of the panels of a set were processed. Following the treatment in each of the baths the sets of panelswere rinsed in cold water for 1 minute and then in hot water at F. for 45 seconds.

Some of the coated panels of each set were tested for their bare corrosion resistance and others of the. coated panels of each set were painted and tested for their paint receptive properties by subjecting the. painted panels. to salt spray and humidity chambers and adhesion tests as in the previous examples. Still others. of the panels were used for coating weight determinations. The coatings on the aluminum panels were observed to vary in shades from dark grey to golden brown and on the steel panels the coatings were slightly dusty generally with some iridescense. On both the aluminum and steel panels, the paint-base properties of the coatings ranged from very good to excellent with the coated aluminum panels test ing better generally than the coated steel panels. All of the panels treated showed excellent adhesion of the coating under impact and after visual observation upon scraping the painted panels with a sharp knife edge. On all of the panels the coatings produced at 0.05% by weight of the condensed sodium phosphate compound were superior to the coatings produced at twice this concentration and particularly was this observed with respect to the corrosion resistance of the coatings produced from the baths having the lower of the two concentrations of condensed phosphate of sodium compound. Coatings produced from the baths containing tetra-sodium pyrophosphate and disodium pyrophosphate at a concentration of 0.05 by weight per total volume produced the best appearing coatings from the standpoint of uniformity and absence of dust. The average coating weight determination for panels processed in the above baths is as follows:

What is claimed is:

1. An aqueous acidic solution for producing corrosionresistant and paint-base coatings on metallic surfaces consisting. essentially of water, at least one compound containing hexavalent chromium, at least one compound containing the fluoride radical, and at least one condensed phosphate compound which yields the condensed phos phate radical in solution and in which the mole ratio of cationic oxides to anionic oxides is greater than 0 and less' than 3, said condensed phosphate compound being present in amounts up to about 0.5% by weight calculated as condensed phosphate of sodium compound.

2. An aqueous acidic solution for producing corrosion; resistant and paint-base coatings on metallic surfaces consisting essentially of water, at least one compound containing hexavalent chromium, at least one compound containing the fluoride radical, and at least one alkali metal condensed phosphate compound in which the mole ratio s 9 I of cationic oxides to anionic oxides is greater than and less than 3, said alkali metal condensed phosphate compound being present in amounts up to about 0.5% by weight calculated as condensed phosphate of sodium compound.

3. An aqueous acidic solution for producing corrosionresistant and paint-base coatings on metallic surfaces consisting essentially of water, between about 0.08% and 2.5% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate, said condensed phosphate compound being present in amounts up to about 0.5%.

4. An aqueous acidic solution for producing corrosionresistant and paint-base coatings on metallic surfaces consisting essentially of water, between about 0.08% and about 2.5% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, up to about 1.6% fluoborate ion expressed as sodium fluoborate, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate, said condensed phosphate compound being present in amounts from between about 0.01% and about 0.5%.

5. An aqueous acidic solution for producing corrosion resistant and paint base coatings on metallic surfaces con sisting essentially of water, between about 0.08% and about 2.5% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, between about 0.6% and about 0.7% fluoborate ion expressed as sodium fluoborate, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate, said condensed phosphate compound being present in amounts from between about 0.01% and about 0.5

6. An aqueous acidic solution for producing corrosion resistant and paint base coatings on metallic surfaces consisting essentially of water, between about 0.3% and about 0.6% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, between about 0.6% and about 0.7% fluoborate ion expressedas sodium fluoborate, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaposphate, said condensed phosphate compound being present in amounts from between about 0.01% and about 0.5

7. An aqueous acidic solution as claimed in claim 4 wherein said solution contains in addition at least one complex fluoride selected from the group consisting of potassium titanium fluoride and potassium zirconium fluoride in a concentration up to about 0.1%

8. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, at least one compound containing hexavalent chromium, at least one compound containing the fluoride radical, and at least one condensed phosphate compound which yields the condensed phosphate radical insolution and in which the mole ratio of cationic oxides to anionic oxides is greater than 0 and less than 3, said condensed phosphate compound being present in amounts up to about 0.5% by weight calculated as condensed phosphate of sodium compound, maintaining said solution in contact with said surface for a suflicient time to form said coating.

9. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, at least gne compound containing hexavalent chromium, at least one compound containing the fluoride radical, and at least one alkali metal condensed phosphate compound in which the mole ratio of cationic oxides to anionic oxides is greater than 0 and less than 3, said alkali metal condensed phosphate compound being present in amounts up to about 0.5% by weight calculated as condensed phosphate of sodium compound, maintaining said solution in contact with said surface for a suflicient time to form said coating.

10. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, between about 0.08% and 2.5% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodiumv pyrophosphate, sodium tripolyphopshate, and sodium hexametaphosphate, said condensed phosphate compound being present in amounts up to about 0.5%,

' maintaining said solution in contact with said surface for a suflicient time to form said coating.

11. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, between about 0.08% and about 2.5% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, up to about 1.6% fluoborate ion expressed as sodium fluoborate, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate, said condensed phosphate compound being present in amounts from between about 0.01% and about 0.5%, maintaining said solution in contact with said surface for a suflicient time to form said coating.

12. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, between about 0.08% and about 2.5 CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, between about 0.6% and about 0.7% fluoborate ion expressed as sodium fluoborate, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate, said condensed phosphate compound being present in amounts from between about 0.01% and about 0.5 maintaining said solution in contact with said surface for a sufficient time to form said coating.

13. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, between about 0.3% and about 0.6% CrO between about 0.01% and about 2% flouride expressed as sodium bifluoride, between about 0.6% and about 0.7% fluoborate ion expressed as sodium fluoborate, and at least one condensed phosphate compound selected from the group consisting of disodium pyrophosphate, tetrasodium pyrophosphate,

' sodium tripolyphosphate, and sodium hexametaphosphate,

said condensed phosphate compound being present in amounts from between about 0.01% and about 0.5%, maintaining said solution in contact with said surface for a sufficient time to form said coating.

14. A method for producing corrosion resistant and paint base coatings on a metallic surface which comprises the steps of contacting the metal surface with an aqueous acidic solution consisting essentially of water, between about 0.08% and about 2.5% CrO between about 0.01% and about 2% fluoride expressed as sodium bifluoride, up to about 1.6% fluoborate ion expressed as sodium fluoborate, at least one complex fluoride selected 11 from the group consisting of potassium titaniumfluoride and potassium zirconiu n'fiuoridetin a concentration up to about '0.1'%,tand at ileast one condensed phosphate compound selected "from 'the,gr,oup consisting of fdisodium pyrop'hosphate, tetrasodium pyrophosphate, sodium .tfipolyphosphate, and sodium hexametaphosphate, saidcondensed phosphate compound being ,present in amounts from between about 0.01 and about 0.5%, maintaining said solution in contact iwitl i saidsurface for a-suffieient time to form said coating.

References Cited in the file of this upatent UNIT ED STATES "PATENTS 

1. AN AQUEOUS ACIDIC SOLUTION FOR PRODUCING CORROSIONRESISTANT AND PAINT-BASE COATINGS ON METALLIC SURFCES CONSISTING ESSENTIALLY OF WATER, AT LEAST ONE COMPOUND CONTAINING HEXAVALENT CHROMIUM, AT LEAST ONE COMPOUND CONTAINING THE FLUORIDE RADICAL, AND AT LEAST ONE CONDENSED PHOSPHATE COMPOUND WHICH YIELDS THE CONDENSED PHOSPHATE RADICAL IN SOLUTION AND IN WHICH THE MOLE RATIO OF CATIONIC OXIDES TO ANIONIC OXIDES IS GREATER THAN 0 AND LESS THAN 3 SAID CONDENSED PHOSPHATE COMPOUND BEING PRESENT IN AMOUNTS UP TO ABOUT 0.5% BY WEIGHT CALCULATED AS CONDENSED PHOSPHATE OF SODIUM COMPOUND. 